Optical Amplifiers, such as the well-known Erbium Doped amplifier, are pumped by a laser source, typically a laser diode, to derive signal gain. Conventional optical amplifier systems implement power control of the amplifier by changing the DC drive current applied to the pump laser diode so as to control the pump power supplied to a gain medium. This method is used to ensure that the optical amplifier operates at a certain power set point. Unfortunately, when the set point is such that the pump laser is operated near its lasing threshold, reflections in the optical path (from anywhere between the pump and the gain medium) can cause drastic fluctuations in the pump operation. When the pump laser is operated close to its threshold, such reflections and other perturbing effects may cause the pump laser to either drop below threshold or to change its output mode. Such fluctuations cause undesirable wide excursions in the pump output power that limit the stability both the pump and the amplifier under low power operation. Additionally, pump bistability may occur at currents significantly above threshold, once again limiting the stability and operation of the amplifier/pump.
Conventional solutions to stabilize the output power of amplifier pump lasers include the use of Fiber Bragg Gratings (FBGs) to stabilize pump wavelength and/or power and pump current dither circuits to stabilize pump power. Pump stabilization designs using FBGs alone impose tight pump laser manufacturing specifications on parameters such as front facet reflectivity and pump laser wavelength control, the latter in order to control detuning between the FBG wavelength and the laser wavelength, on the order of a few to several nm. The FBG designs for good power stability also typically require long fiber lengths and one or more FBGs in the fiber pigtail at one or more meters from the pump chip, both leading to higher cost.
Although the use of FBGs provides a stable wavelength of operation for the pump at high output powers, it, unfortunately, does not ensure pump power stability at very low power. As the drive current to an externally stabilized pump laser is increased through and above the lasing threshold, the device will commence laser operation in a single mode external cavity state and will, later, transition to the more stable multi-mode coherence collapse regime. The critical current at which the stable multi-mode operation occurs is determined by many interacting pump parameters. Minimizing this critical current typically requires increasing laser facet reflectivity and decreasing the FBG reflectivity. Unfortunately, however, designs for high power and/or operation over a wide temperature range (so-called “uncooled” pumps) both typically require extremely low laser front facet reflectivity. Low laser facet reflectivity enhances undesirable low power bistability and compounds the design problem. Further, FBG designs for stable power operation at the high power end of the pump operating range require the use of stronger gratings and lower front facet reflectivity, both situations detrimental for low power stabilization. Finally, in an uncooled pump, it is not possible to maintain tight detuning ranges on the order of a few nm, when the native pump wavelength shifts by 25 nm or more.
The conventional dither circuit technique for controlling pump power utilizes a small, continuous variation in the pump current wherein the rate of variation exceeds the excited state lifetime of the amplifier active gain medium. The dither circuit is typically implemented using additional circuit elements such as a bias control circuit. Disadvantageously, this second conventional method of pump control does not improve pump stability at very low power (near threshold) and adds complexity of additional circuit management.
Based upon the above discussions, it is concluded that there is a need, in the art, for an improved system and method for controlling optical amplifier output power. The improved system and method should be capable of controlling the pump laser in such a fashion that the optical amplifier output is stable over a wide range of output powers that includes low powers. The present invention addresses such a need.